bval-jsr/src/main/java/org/apache/bval/util/reflection/TypeUtils.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.bval.util.reflection;

import java.lang.reflect.Array;
import java.lang.reflect.GenericArrayType;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.lang.reflect.TypeVariable;
import java.lang.reflect.WildcardType;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.Set;
import java.util.stream.Stream;

import org.apache.bval.util.ObjectUtils;
import org.apache.bval.util.Validate;

/**
 * Taken from commons-lang3.
 *
 * <p> Utility methods focusing on type inspection, particularly with regard to
 * generics. </p>
 *
 * @since 1.1.2
 */
public class TypeUtils {

    /**
     * {@link WildcardType} builder.
     */
    public static class WildcardTypeBuilder {
        /**
         * Constructor
         */
        private WildcardTypeBuilder() {
        }
        
        private Type[] upperBounds;
        private Type[] lowerBounds;

        /**
         * Specify upper bounds of the wildcard type to build.
         * @param bounds to set
         * @return {@code this}
         */
        public WildcardTypeBuilder withUpperBounds(final Type... bounds) {
            this.upperBounds = bounds;
            return this;
        }

        /**
         * Specify lower bounds of the wildcard type to build.
         * @param bounds to set
         * @return {@code this}
         */
        public WildcardTypeBuilder withLowerBounds(final Type... bounds) {
            this.lowerBounds = bounds;
            return this;
        }

        /**
         * {@inheritDoc}
         */
        public WildcardType build() {
            return new WildcardTypeImpl(upperBounds, lowerBounds);
        }
    }

    /**
     * ParameterizedType implementation class.
     */
    private static final class ParameterizedTypeImpl implements ParameterizedType {
        private final Class<?> raw;
        private final Type useOwner;
        private final Type[] typeArguments;

        /**
         * Constructor
         * @param raw type
         * @param useOwner owner type to use, if any
         * @param typeArguments formal type arguments
         */
        private ParameterizedTypeImpl(final Class<?> raw, final Type useOwner, final Type[] typeArguments) {
            this.raw = raw;
            this.useOwner = useOwner;
            this.typeArguments = typeArguments.clone();
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public Type getRawType() {
            return raw;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public Type getOwnerType() {
            return useOwner;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public Type[] getActualTypeArguments() {
            return typeArguments.clone();
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public String toString() {
            return TypeUtils.toString(this);
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public boolean equals(final Object obj) {
            return obj == this || obj instanceof ParameterizedType && TypeUtils.equals(this, ((ParameterizedType) obj));
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public int hashCode() {
            return Objects.hash(raw, useOwner, typeArguments);
        }
    }

    /**
     * WildcardType implementation class.
     */
    private static final class WildcardTypeImpl implements WildcardType {
        private static final Type[] EMPTY_UPPER_BOUNDS = { Object.class };
        private static final Type[] EMPTY_LOWER_BOUNDS = new Type[0];

        private final Type[] upperBounds;
        private final Type[] lowerBounds;

        /**
         * Constructor
         * @param upperBounds of this type
         * @param lowerBounds of this type
         */
        private WildcardTypeImpl(final Type[] upperBounds, final Type[] lowerBounds) {
            this.upperBounds = ObjectUtils.isEmptyArray(upperBounds) ? EMPTY_UPPER_BOUNDS : upperBounds;
            this.lowerBounds = lowerBounds == null ? EMPTY_LOWER_BOUNDS : lowerBounds;
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public Type[] getUpperBounds() {
            return upperBounds.clone();
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public Type[] getLowerBounds() {
            return lowerBounds.clone();
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public String toString() {
            return TypeUtils.toString(this);
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public boolean equals(final Object obj) {
            return obj == this || obj instanceof WildcardType && TypeUtils.equals(this, (WildcardType) obj);
        }

        /**
         * {@inheritDoc}
         */
        @Override
        public int hashCode() {
            return Objects.hash(upperBounds, lowerBounds);
        }
    }

    /**
     * <p>{@code TypeUtils} instances should NOT be constructed in standard
     * programming. Instead, the class should be used as
     * {@code TypeUtils.isAssignable(cls, toClass)}.</p> <p>This
     * constructor is public to permit tools that require a JavaBean instance to
     * operate.</p>
     */
    private TypeUtils() {
    }

    /**
     * <p>Checks if the subject type may be implicitly cast to the target type
     * following the Java generics rules. If both types are {@link Class}
     * objects, the method returns the result of
     * {@link Class#isAssignableFrom(Class)}.</p>
     *
     * @param type the subject type to be assigned to the target type
     * @param toType the target type
     * @return {@code true} if {@code type} is assignable to {@code toType}.
     */
    public static boolean isAssignable(final Type type, final Type toType) {
        return isAssignable(type, toType, null);
    }

    /**
     * <p>Checks if the subject type may be implicitly cast to the target type
     * following the Java generics rules.</p>
     *
     * @param type the subject type to be assigned to the target type
     * @param toType the target type
     * @param typeVarAssigns optional map of type variable assignments
     * @return {@code true} if {@code type} is assignable to {@code toType}.
     */
    private static boolean isAssignable(final Type type, final Type toType,
            final Map<TypeVariable<?>, Type> typeVarAssigns) {
        if (toType == null || toType instanceof Class<?>) {
            return isAssignable(type, (Class<?>) toType);
        }

        if (toType instanceof ParameterizedType) {
            return isAssignable(type, (ParameterizedType) toType, typeVarAssigns);
        }

        if (toType instanceof GenericArrayType) {
            return isAssignable(type, (GenericArrayType) toType, typeVarAssigns);
        }

        if (toType instanceof WildcardType) {
            return isAssignable(type, (WildcardType) toType, typeVarAssigns);
        }

        if (toType instanceof TypeVariable<?>) {
            return isAssignable(type, (TypeVariable<?>) toType, typeVarAssigns);
        }

        throw new IllegalStateException("found an unhandled type: " + toType);
    }

    /**
     * <p>Checks if the subject type may be implicitly cast to the target class
     * following the Java generics rules.</p>
     *
     * @param type the subject type to be assigned to the target type
     * @param toClass the target class
     * @return {@code true} if {@code type} is assignable to {@code toClass}.
     */
    private static boolean isAssignable(final Type type, final Class<?> toClass) {
        if (type == null) {
            // consistency with ClassUtils.isAssignable() behavior
            return toClass == null || !toClass.isPrimitive();
        }

        // only a null type can be assigned to null type which
        // would have cause the previous to return true
        if (toClass == null) {
            return false;
        }

        // all types are assignable to themselves
        if (toClass.equals(type)) {
            return true;
        }

        if (type instanceof Class<?>) {
            // just comparing two classes
            // also take primitives into account!
            return isAssignable((Class<?>) type, toClass);
        }

        if (type instanceof ParameterizedType) {
            // only have to compare the raw type to the class
            return isAssignable(getRawType((ParameterizedType) type), toClass);
        }

        // *
        if (type instanceof TypeVariable<?>) {
            // if any of the bounds are assignable to the class, then the
            // type is assignable to the class.
            return Stream.of(((TypeVariable<?>) type).getBounds()).anyMatch(bound -> isAssignable(bound, toClass));
        }

        // the only classes to which a generic array type can be assigned
        // are class Object and array classes
        if (type instanceof GenericArrayType) {
            return Object.class.equals(toClass)
                    || toClass.isArray()
                    && isAssignable(((GenericArrayType) type).getGenericComponentType(), toClass
                            .getComponentType());
        }

        // wildcard types are not assignable to a class (though one would think
        // "? super Object" would be assignable to Object)
        if (type instanceof WildcardType) {
            return false;
        }

        throw new IllegalStateException("found an unhandled type: " + type);
    }

    private static boolean isAssignable(Class<?> cls, final Class<?> toClass) {
        if (toClass == null) {
            return false;
        }
        // have to check for null, as isAssignableFrom doesn't
        if (cls == null) {
            return !toClass.isPrimitive();
        }

        if (cls.isPrimitive() && !toClass.isPrimitive()) {
            cls = Reflection.primitiveToWrapper(cls);
            if (cls == null) {
                return false;
            }
        }
        if (toClass.isPrimitive() && !cls.isPrimitive()) {
            cls = Reflection.wrapperToPrimitive(cls);
            if (cls == null) {
                return false;
            }
        }
        if (cls.equals(toClass)) {
            return true;
        }
        if (cls.isPrimitive()) {
            if (toClass.isPrimitive() == false) {
                return false;
            }
            if (Integer.TYPE.equals(cls)) {
                return Long.TYPE.equals(toClass)
                        || Float.TYPE.equals(toClass)
                        || Double.TYPE.equals(toClass);
            }
            if (Long.TYPE.equals(cls)) {
                return Float.TYPE.equals(toClass)
                        || Double.TYPE.equals(toClass);
            }
            if (Boolean.TYPE.equals(cls)) {
                return false;
            }
            if (Double.TYPE.equals(cls)) {
                return false;
            }
            if (Float.TYPE.equals(cls)) {
                return Double.TYPE.equals(toClass);
            }
            if (Character.TYPE.equals(cls)) {
                return Integer.TYPE.equals(toClass)
                        || Long.TYPE.equals(toClass)
                        || Float.TYPE.equals(toClass)
                        || Double.TYPE.equals(toClass);
            }
            if (Short.TYPE.equals(cls)) {
                return Integer.TYPE.equals(toClass)
                        || Long.TYPE.equals(toClass)
                        || Float.TYPE.equals(toClass)
                        || Double.TYPE.equals(toClass);
            }
            if (Byte.TYPE.equals(cls)) {
                return Short.TYPE.equals(toClass)
                        || Integer.TYPE.equals(toClass)
                        || Long.TYPE.equals(toClass)
                        || Float.TYPE.equals(toClass)
                        || Double.TYPE.equals(toClass);
            }
            // should never get here
            return false;
        }
        return toClass.isAssignableFrom(cls);

    }

    /**
     * <p>Checks if the subject type may be implicitly cast to the target
     * parameterized type following the Java generics rules.</p>
     *
     * @param type the subject type to be assigned to the target type
     * @param toParameterizedType the target parameterized type
     * @param typeVarAssigns a map with type variables
     * @return {@code true} if {@code type} is assignable to {@code toType}.
     */
    private static boolean isAssignable(final Type type, final ParameterizedType toParameterizedType,
            final Map<TypeVariable<?>, Type> typeVarAssigns) {
        if (type == null) {
            return true;
        }

        // only a null type can be assigned to null type which
        // would have cause the previous to return true
        if (toParameterizedType == null) {
            return false;
        }

        // all types are assignable to themselves
        if (toParameterizedType.equals(type)) {
            return true;
        }

        // get the target type's raw type
        final Class<?> toClass = getRawType(toParameterizedType);
        // get the subject type's type arguments including owner type arguments
        // and supertype arguments up to and including the target class.
        final Map<TypeVariable<?>, Type> fromTypeVarAssigns = getTypeArguments(type, toClass, null);

        // null means the two types are not compatible
        if (fromTypeVarAssigns == null) {
            return false;
        }

        // compatible types, but there's no type arguments. this is equivalent
        // to comparing Map< ?, ? > to Map, and raw types are always assignable
        // to parameterized types.
        if (fromTypeVarAssigns.isEmpty()) {
            return true;
        }

        // get the target type's type arguments including owner type arguments
        final Map<TypeVariable<?>, Type> toTypeVarAssigns = getTypeArguments(toParameterizedType,
                toClass, typeVarAssigns);

        // now to check each type argument
        for (final TypeVariable<?> var : toTypeVarAssigns.keySet()) {
            final Type toTypeArg = unrollVariableAssignments(var, toTypeVarAssigns);
            final Type fromTypeArg = unrollVariableAssignments(var, fromTypeVarAssigns);

            if (toTypeArg == null && fromTypeArg instanceof Class) {
                continue;
            }

            // parameters must either be absent from the subject type, within
            // the bounds of the wildcard type, or be an exact match to the
            // parameters of the target type.
            if (fromTypeArg != null
                    && !toTypeArg.equals(fromTypeArg)
                    && !(toTypeArg instanceof WildcardType && isAssignable(fromTypeArg, toTypeArg,
                            typeVarAssigns))) {
                return false;
            }
        }
        return true;
    }

    /**
     * Look up {@code var} in {@code typeVarAssigns} <em>transitively</em>,
     * i.e. keep looking until the value found is <em>not</em> a type variable.
     * @param var the type variable to look up
     * @param typeVarAssigns the map used for the look up
     * @return Type or {@code null} if some variable was not in the map
     */
    private static Type unrollVariableAssignments(TypeVariable<?> var, final Map<TypeVariable<?>, Type> typeVarAssigns) {
        Type result;
        do {
            result = typeVarAssigns.get(var);
            if (result instanceof TypeVariable<?> && !result.equals(var)) {
                var = (TypeVariable<?>) result;
                continue;
            }
            break;
        } while (true);
        return result;
    }

    /**
     * <p>Checks if the subject type may be implicitly cast to the target
     * generic array type following the Java generics rules.</p>
     *
     * @param type the subject type to be assigned to the target type
     * @param toGenericArrayType the target generic array type
     * @param typeVarAssigns a map with type variables
     * @return {@code true} if {@code type} is assignable to
     * {@code toGenericArrayType}.
     */
    private static boolean isAssignable(final Type type, final GenericArrayType toGenericArrayType,
            final Map<TypeVariable<?>, Type> typeVarAssigns) {
        if (type == null) {
            return true;
        }

        // only a null type can be assigned to null type which
        // would have cause the previous to return true
        if (toGenericArrayType == null) {
            return false;
        }

        // all types are assignable to themselves
        if (toGenericArrayType.equals(type)) {
            return true;
        }

        final Type toComponentType = toGenericArrayType.getGenericComponentType();

        if (type instanceof Class<?>) {
            final Class<?> cls = (Class<?>) type;

            // compare the component types
            return cls.isArray()
                    && isAssignable(cls.getComponentType(), toComponentType, typeVarAssigns);
        }

        if (type instanceof GenericArrayType) {
            // compare the component types
            return isAssignable(((GenericArrayType) type).getGenericComponentType(),
                    toComponentType, typeVarAssigns);
        }

        if (type instanceof WildcardType) {
            // so long as one of the upper bounds is assignable, it's good
            return Stream.of(getImplicitUpperBounds((WildcardType) type))
                .anyMatch(bound -> isAssignable(bound, toGenericArrayType));
        }

        if (type instanceof TypeVariable<?>) {
            // probably should remove the following logic and just return false.
            // type variables cannot specify arrays as bounds.
            return Stream.of(getImplicitBounds((TypeVariable<?>) type))
                .anyMatch(bound -> isAssignable(bound, toGenericArrayType));
        }

        if (type instanceof ParameterizedType) {
            // the raw type of a parameterized type is never an array or
            // generic array, otherwise the declaration would look like this:
            // Collection[]< ? extends String > collection;
            return false;
        }

        throw new IllegalStateException("found an unhandled type: " + type);
    }

    /**
     * <p>Checks if the subject type may be implicitly cast to the target
     * wildcard type following the Java generics rules.</p>
     *
     * @param type the subject type to be assigned to the target type
     * @param toWildcardType the target wildcard type
     * @param typeVarAssigns a map with type variables
     * @return {@code true} if {@code type} is assignable to
     * {@code toWildcardType}.
     */
    private static boolean isAssignable(final Type type, final WildcardType toWildcardType,
            final Map<TypeVariable<?>, Type> typeVarAssigns) {
        if (type == null) {
            return true;
        }

        // only a null type can be assigned to null type which
        // would have cause the previous to return true
        if (toWildcardType == null) {
            return false;
        }

        // all types are assignable to themselves
        if (toWildcardType.equals(type)) {
            return true;
        }

        final Type[] toUpperBounds = getImplicitUpperBounds(toWildcardType);
        final Type[] toLowerBounds = getImplicitLowerBounds(toWildcardType);

        if (type instanceof WildcardType) {
            final WildcardType wildcardType = (WildcardType) type;
            final Type[] upperBounds = getImplicitUpperBounds(wildcardType);
            final Type[] lowerBounds = getImplicitLowerBounds(wildcardType);

            for (Type toBound : toUpperBounds) {
                // if there are assignments for unresolved type variables,
                // now's the time to substitute them.
                toBound = substituteTypeVariables(toBound, typeVarAssigns);

                // each upper bound of the subject type has to be assignable to
                // each
                // upper bound of the target type
                for (final Type bound : upperBounds) {
                    if (!isAssignable(bound, toBound, typeVarAssigns)) {
                        return false;
                    }
                }
            }

            for (Type toBound : toLowerBounds) {
                // if there are assignments for unresolved type variables,
                // now's the time to substitute them.
                toBound = substituteTypeVariables(toBound, typeVarAssigns);

                // each lower bound of the target type has to be assignable to
                // each
                // lower bound of the subject type
                for (final Type bound : lowerBounds) {
                    if (!isAssignable(toBound, bound, typeVarAssigns)) {
                        return false;
                    }
                }
            }
            return true;
        }

        for (final Type toBound : toUpperBounds) {
            // if there are assignments for unresolved type variables,
            // now's the time to substitute them.
            if (!isAssignable(type, substituteTypeVariables(toBound, typeVarAssigns),
                    typeVarAssigns)) {
                return false;
            }
        }

        for (final Type toBound : toLowerBounds) {
            // if there are assignments for unresolved type variables,
            // now's the time to substitute them.
            if (!isAssignable(substituteTypeVariables(toBound, typeVarAssigns), type,
                    typeVarAssigns)) {
                return false;
            }
        }
        return true;
    }

    /**
     * <p>Checks if the subject type may be implicitly cast to the target type
     * variable following the Java generics rules.</p>
     *
     * @param type the subject type to be assigned to the target type
     * @param toTypeVariable the target type variable
     * @param typeVarAssigns a map with type variables
     * @return {@code true} if {@code type} is assignable to
     * {@code toTypeVariable}.
     */
    private static boolean isAssignable(final Type type, final TypeVariable<?> toTypeVariable,
            final Map<TypeVariable<?>, Type> typeVarAssigns) {
        if (type == null) {
            return true;
        }

        // only a null type can be assigned to null type which
        // would have cause the previous to return true
        if (toTypeVariable == null) {
            return false;
        }

        // all types are assignable to themselves
        if (toTypeVariable.equals(type)) {
            return true;
        }

        if (type instanceof TypeVariable<?>) {
            // a type variable is assignable to another type variable, if
            // and only if the former is the latter, extends the latter, or
            // is otherwise a descendant of the latter.
            final Type[] bounds = getImplicitBounds((TypeVariable<?>) type);

            for (final Type bound : bounds) {
                if (isAssignable(bound, toTypeVariable, typeVarAssigns)) {
                    return true;
                }
            }
        }

        if (type instanceof Class<?> || type instanceof ParameterizedType
                || type instanceof GenericArrayType || type instanceof WildcardType) {
            return false;
        }

        throw new IllegalStateException("found an unhandled type: " + type);
    }

    /**
     * <p>Find the mapping for {@code type} in {@code typeVarAssigns}.</p>
     *
     * @param type the type to be replaced
     * @param typeVarAssigns the map with type variables
     * @return the replaced type
     * @throws IllegalArgumentException if the type cannot be substituted
     */
    private static Type substituteTypeVariables(final Type type, final Map<TypeVariable<?>, Type> typeVarAssigns) {
        if (type instanceof TypeVariable<?> && typeVarAssigns != null) {
            final Type replacementType = typeVarAssigns.get(type);

            if (replacementType == null) {
                throw new IllegalArgumentException("missing assignment type for type variable "
                        + type);
            }
            return replacementType;
        }
        return type;
    }

    /**
     * <p>Retrieves all the type arguments for this parameterized type
     * including owner hierarchy arguments such as
     * {@code Outer<K,V>.Inner<T>.DeepInner<E>} .
     * The arguments are returned in a
     * {@link Map} specifying the argument type for each {@link TypeVariable}.
     * </p>
     *
     * @param type specifies the subject parameterized type from which to
     *             harvest the parameters.
     * @return a {@code Map} of the type arguments to their respective type
     * variables.
     */
    public static Map<TypeVariable<?>, Type> getTypeArguments(final ParameterizedType type) {
        return getTypeArguments(type, getRawType(type), null);
    }

    /**
     * <p>Gets the type arguments of a class/interface based on a subtype. For
     * instance, this method will determine that both of the parameters for the
     * interface {@link Map} are {@link Object} for the subtype
     * {@link java.util.Properties Properties} even though the subtype does not
     * directly implement the {@code Map} interface.</p>
     * <p>This method returns {@code null} if {@code type} is not assignable to
     * {@code toClass}. It returns an empty map if none of the classes or
     * interfaces in its inheritance hierarchy specify any type arguments.</p>
     * <p>A side effect of this method is that it also retrieves the type
     * arguments for the classes and interfaces that are part of the hierarchy
     * between {@code type} and {@code toClass}. So with the above
     * example, this method will also determine that the type arguments for
     * {@link java.util.Hashtable Hashtable} are also both {@code Object}.
     * In cases where the interface specified by {@code toClass} is
     * (indirectly) implemented more than once (e.g. where {@code toClass}
     * specifies the interface {@link Iterable Iterable} and
     * {@code type} specifies a parameterized type that implements both
     * {@link Set Set} and {@link java.util.Collection Collection}),
     * this method will look at the inheritance hierarchy of only one of the
     * implementations/subclasses; the first interface encountered that isn't a
     * subinterface to one of the others in the {@code type} to
     * {@code toClass} hierarchy.</p>
     *
     * @param type the type from which to determine the type parameters of
     * {@code toClass}
     * @param toClass the class whose type parameters are to be determined based
     * on the subtype {@code type}
     * @return a {@code Map} of the type assignments for the type variables in
     * each type in the inheritance hierarchy from {@code type} to
     * {@code toClass} inclusive.
     */
    public static Map<TypeVariable<?>, Type> getTypeArguments(final Type type, final Class<?> toClass) {
        return getTypeArguments(type, toClass, null);
    }

    /**
     * <p>Return a map of the type arguments of @{code type} in the context of {@code toClass}.</p>
     *
     * @param type the type in question
     * @param toClass the class
     * @param subtypeVarAssigns a map with type variables
     * @return the {@code Map} with type arguments
     */
    private static Map<TypeVariable<?>, Type> getTypeArguments(final Type type, final Class<?> toClass,
            final Map<TypeVariable<?>, Type> subtypeVarAssigns) {
        if (type instanceof Class<?>) {
            return getTypeArguments((Class<?>) type, toClass, subtypeVarAssigns);
        }

        if (type instanceof ParameterizedType) {
            return getTypeArguments((ParameterizedType) type, toClass, subtypeVarAssigns);
        }

        if (type instanceof GenericArrayType) {
            return getTypeArguments(((GenericArrayType) type).getGenericComponentType(), toClass
                    .isArray() ? toClass.getComponentType() : toClass, subtypeVarAssigns);
        }

        // since wildcard types are not assignable to classes, should this just
        // return null?
        if (type instanceof WildcardType) {
            for (final Type bound : getImplicitUpperBounds((WildcardType) type)) {
                // find the first bound that is assignable to the target class
                if (isAssignable(bound, toClass)) {
                    return getTypeArguments(bound, toClass, subtypeVarAssigns);
                }
            }

            return null;
        }

        if (type instanceof TypeVariable<?>) {
            for (final Type bound : getImplicitBounds((TypeVariable<?>) type)) {
                // find the first bound that is assignable to the target class
                if (isAssignable(bound, toClass)) {
                    return getTypeArguments(bound, toClass, subtypeVarAssigns);
                }
            }

            return null;
        }
        throw new IllegalStateException("found an unhandled type: " + type);
    }

    /**
     * <p>Return a map of the type arguments of a parameterized type in the context of {@code toClass}.</p>
     *
     * @param parameterizedType the parameterized type
     * @param toClass the class
     * @param subtypeVarAssigns a map with type variables
     * @return the {@code Map} with type arguments
     */
    private static Map<TypeVariable<?>, Type> getTypeArguments(
            final ParameterizedType parameterizedType, final Class<?> toClass,
            final Map<TypeVariable<?>, Type> subtypeVarAssigns) {
        final Class<?> cls = getRawType(parameterizedType);

        // make sure they're assignable
        if (!isAssignable(cls, toClass)) {
            return null;
        }

        final Type ownerType = parameterizedType.getOwnerType();
        Map<TypeVariable<?>, Type> typeVarAssigns;

        if (ownerType instanceof ParameterizedType) {
            // get the owner type arguments first
            final ParameterizedType parameterizedOwnerType = (ParameterizedType) ownerType;
            typeVarAssigns = getTypeArguments(parameterizedOwnerType,
                    getRawType(parameterizedOwnerType), subtypeVarAssigns);
        } else {
            // no owner, prep the type variable assignments map
            typeVarAssigns = subtypeVarAssigns == null ? new HashMap<>() : new HashMap<>(subtypeVarAssigns);
        }

        // get the subject parameterized type's arguments
        final Type[] typeArgs = parameterizedType.getActualTypeArguments();
        // and get the corresponding type variables from the raw class
        final TypeVariable<?>[] typeParams = cls.getTypeParameters();

        // map the arguments to their respective type variables
        for (int i = 0; i < typeParams.length; i++) {
            final Type typeArg = typeArgs[i];
            typeVarAssigns.put(typeParams[i], typeVarAssigns.containsKey(typeArg) ? typeVarAssigns
                    .get(typeArg) : typeArg);
        }

        if (toClass.equals(cls)) {
            // target class has been reached. Done.
            return typeVarAssigns;
        }

        // walk the inheritance hierarchy until the target class is reached
        return getTypeArguments(getClosestParentType(cls, toClass), toClass, typeVarAssigns);
    }

    /**
     * <p>Return a map of the type arguments of a class in the context of @{code toClass}.</p>
     *
     * @param cls the class in question
     * @param toClass the context class
     * @param subtypeVarAssigns a map with type variables
     * @return the {@code Map} with type arguments
     */
    private static Map<TypeVariable<?>, Type> getTypeArguments(Class<?> cls, final Class<?> toClass,
            final Map<TypeVariable<?>, Type> subtypeVarAssigns) {
        // make sure they're assignable
        if (!isAssignable(cls, toClass)) {
            return null;
        }

        // can't work with primitives
        if (cls.isPrimitive()) {
            // both classes are primitives?
            if (toClass.isPrimitive()) {
                // dealing with widening here. No type arguments to be
                // harvested with these two types.
                return new HashMap<>();
            }

            // work with wrapper the wrapper class instead of the primitive
            cls = Reflection.primitiveToWrapper(cls);
        }

        // create a copy of the incoming map, or an empty one if it's null
        final Map<TypeVariable<?>, Type> typeVarAssigns =
            subtypeVarAssigns == null ? new HashMap<>() : new HashMap<>(subtypeVarAssigns);

        // has target class been reached?
        if (toClass.equals(cls)) {
            return typeVarAssigns;
        }

        // walk the inheritance hierarchy until the target class is reached
        return getTypeArguments(getClosestParentType(cls, toClass), toClass, typeVarAssigns);
    }

    /**
     * <p>Get the closest parent type to the
     * super class specified by {@code superClass}.</p>
     *
     * @param cls the class in question
     * @param superClass the super class
     * @return the closes parent type
     */
    private static Type getClosestParentType(final Class<?> cls, final Class<?> superClass) {
        // only look at the interfaces if the super class is also an interface
        if (superClass.isInterface()) {
            // get the generic interfaces of the subject class
            final Type[] interfaceTypes = cls.getGenericInterfaces();
            // will hold the best generic interface match found
            Type genericInterface = null;

            // find the interface closest to the super class
            for (final Type midType : interfaceTypes) {
                Class<?> midClass = null;

                if (midType instanceof ParameterizedType) {
                    midClass = getRawType((ParameterizedType) midType);
                } else if (midType instanceof Class<?>) {
                    midClass = (Class<?>) midType;
                } else {
                    throw new IllegalStateException("Unexpected generic"
                            + " interface type found: " + midType);
                }

                // check if this interface is further up the inheritance chain
                // than the previously found match
                if (isAssignable(midClass, superClass)
                        && isAssignable(genericInterface, (Type) midClass)) {
                    genericInterface = midType;
                }
            }

            // found a match?
            if (genericInterface != null) {
                return genericInterface;
            }
        }

        // none of the interfaces were descendants of the target class, so the
        // super class has to be one, instead
        return cls.getGenericSuperclass();
    }

    /**
     * <p>Checks if the given value can be assigned to the target type
     * following the Java generics rules.</p>
     *
     * @param value the value to be checked
     * @param type the target type
     * @return {@code true} if {@code value} is an instance of {@code type}.
     */
    public static boolean isInstance(final Object value, final Type type) {
        if (type == null) {
            return false;
        }

        return value == null ? !(type instanceof Class<?>) || type == void.class || !((Class<?>) type).isPrimitive()
                : isAssignable(value.getClass(), type, null);
    }

    /**
     * <p>This method strips out the redundant upper bound types in type
     * variable types and wildcard types (or it would with wildcard types if
     * multiple upper bounds were allowed).</p> <p>Example, with the variable
     * type declaration:
     *
     * <pre>&lt;K extends java.util.Collection&lt;String&gt; &amp;
     * java.util.List&lt;String&gt;&gt;</pre>
     *
     * <p>
     * since {@code List} is a subinterface of {@code Collection},
     * this method will return the bounds as if the declaration had been:
     * </p>
     *
     * <pre>&lt;K extends java.util.List&lt;String&gt;&gt;</pre>
     *
     * @param bounds an array of types representing the upper bounds of either
     * {@link WildcardType} or {@link TypeVariable}, not {@code null}.
     * @return an array containing the values from {@code bounds} minus the
     * redundant types.
     */
    public static Type[] normalizeUpperBounds(final Type[] bounds) {
        Validate.notNull(bounds, "null value specified for bounds array");
        // don't bother if there's only one (or none) type
        if (bounds.length < 2) {
            return bounds;
        }

        final Set<Type> types = new HashSet<>(bounds.length);

        for (final Type type1 : bounds) {
            boolean subtypeFound = false;

            for (final Type type2 : bounds) {
                if (type1 != type2 && isAssignable(type2, type1, null)) {
                    subtypeFound = true;
                    break;
                }
            }

            if (!subtypeFound) {
                types.add(type1);
            }
        }

        return types.toArray(new Type[types.size()]);
    }

    /**
     * <p>Returns an array containing the sole type of {@link Object} if
     * {@link TypeVariable#getBounds()} returns an empty array. Otherwise, it
     * returns the result of {@link TypeVariable#getBounds()} passed into
     * {@link #normalizeUpperBounds}.</p>
     *
     * @param typeVariable the subject type variable, not {@code null}
     * @return a non-empty array containing the bounds of the type variable.
     */
    public static Type[] getImplicitBounds(final TypeVariable<?> typeVariable) {
        Validate.notNull(typeVariable, "typeVariable is null");
        final Type[] bounds = typeVariable.getBounds();

        return bounds.length == 0 ? new Type[] { Object.class } : normalizeUpperBounds(bounds);
    }

    /**
     * <p>Returns an array containing the sole value of {@link Object} if
     * {@link WildcardType#getUpperBounds()} returns an empty array. Otherwise,
     * it returns the result of {@link WildcardType#getUpperBounds()}
     * passed into {@link #normalizeUpperBounds}.</p>
     *
     * @param wildcardType the subject wildcard type, not {@code null}
     * @return a non-empty array containing the upper bounds of the wildcard
     * type.
     */
    public static Type[] getImplicitUpperBounds(final WildcardType wildcardType) {
        Validate.notNull(wildcardType, "wildcardType is null");
        final Type[] bounds = wildcardType.getUpperBounds();

        return bounds.length == 0 ? new Type[] { Object.class } : normalizeUpperBounds(bounds);
    }

    /**
     * <p>Returns an array containing a single value of {@code null} if
     * {@link WildcardType#getLowerBounds()} returns an empty array. Otherwise,
     * it returns the result of {@link WildcardType#getLowerBounds()}.</p>
     *
     * @param wildcardType the subject wildcard type, not {@code null}
     * @return a non-empty array containing the lower bounds of the wildcard
     * type.
     */
    public static Type[] getImplicitLowerBounds(final WildcardType wildcardType) {
        Validate.notNull(wildcardType, "wildcardType is null");
        final Type[] bounds = wildcardType.getLowerBounds();

        return bounds.length == 0 ? new Type[] { null } : bounds;
    }

    /**
     * <p>Transforms the passed in type to a {@link Class} object. Type-checking method of convenience.</p>
     *
     * @param parameterizedType the type to be converted
     * @return the corresponding {@code Class} object
     * @throws IllegalStateException if the conversion fails
     */
    private static Class<?> getRawType(final ParameterizedType parameterizedType) {
        final Type rawType = parameterizedType.getRawType();

        // check if raw type is a Class object
        // not currently necessary, but since the return type is Type instead of
        // Class, there's enough reason to believe that future versions of Java
        // may return other Type implementations. And type-safety checking is
        // rarely a bad idea.
        if (!(rawType instanceof Class<?>)) {
            throw new IllegalStateException("Wait... What!? Type of rawType: " + rawType);
        }

        return (Class<?>) rawType;
    }

    /**
     * <p>Get the raw type of a Java type, given its context. Primarily for use
     * with {@link TypeVariable}s and {@link GenericArrayType}s, or when you do
     * not know the runtime type of {@code type}: if you know you have a
     * {@link Class} instance, it is already raw; if you know you have a
     * {@link ParameterizedType}, its raw type is only a method call away.</p>
     *
     * @param type to resolve
     * @param assigningType type to be resolved against
     * @return the resolved {@link Class} object or {@code null} if
     * the type could not be resolved
     */
    public static Class<?> getRawType(final Type type, final Type assigningType) {
        if (type instanceof Class<?>) {
            // it is raw, no problem
            return (Class<?>) type;
        }

        if (type instanceof ParameterizedType) {
            // simple enough to get the raw type of a ParameterizedType
            return getRawType((ParameterizedType) type);
        }

        if (type instanceof TypeVariable<?>) {
            if (assigningType == null) {
                return null;
            }

            // get the entity declaring this type variable
            final Object genericDeclaration = ((TypeVariable<?>) type).getGenericDeclaration();

            // can't get the raw type of a method- or constructor-declared type
            // variable
            if (!(genericDeclaration instanceof Class<?>)) {
                return null;
            }

            // get the type arguments for the declaring class/interface based
            // on the enclosing type
            final Map<TypeVariable<?>, Type> typeVarAssigns = getTypeArguments(assigningType,
                    (Class<?>) genericDeclaration);

            // enclosingType has to be a subclass (or subinterface) of the
            // declaring type
            if (typeVarAssigns == null) {
                return null;
            }

            // get the argument assigned to this type variable
            final Type typeArgument = typeVarAssigns.get(type);

            if (typeArgument == null) {
                return null;
            }

            // get the argument for this type variable
            return getRawType(typeArgument, assigningType);
        }

        if (type instanceof GenericArrayType) {
            // get raw component type
            final Class<?> rawComponentType = getRawType(((GenericArrayType) type)
                    .getGenericComponentType(), assigningType);

            // create array type from raw component type and return its class
            return Array.newInstance(rawComponentType, 0).getClass();
        }

        // (hand-waving) this is not the method you're looking for
        if (type instanceof WildcardType) {
            return null;
        }

        throw new IllegalArgumentException("unknown type: " + type);
    }

    /**
     * Learn whether the specified type denotes an array type.
     * @param type the type to be checked
     * @return {@code true} if {@code type} is an array class or a {@link GenericArrayType}.
     */
    public static boolean isArrayType(final Type type) {
        return type instanceof GenericArrayType || type instanceof Class<?> && ((Class<?>) type).isArray();
    }

    /**
     * Get the array component type of {@code type}.
     * @param type the type to be checked
     * @return component type or null if type is not an array type
     */
    public static Type getArrayComponentType(final Type type) {
        if (type instanceof Class<?>) {
            final Class<?> clazz = (Class<?>) type;
            return clazz.isArray() ? clazz.getComponentType() : null;
        }
        if (type instanceof GenericArrayType) {
            return ((GenericArrayType) type).getGenericComponentType();
        }
        return null;
    }

    /**
     * Get a type representing {@code type} with variable assignments "unrolled."
     *
     * @param typeArguments as from {@link TypeUtils#getTypeArguments(Type, Class)}
     * @param type the type to unroll variable assignments for
     * @return Type
     */
    public static Type unrollVariables(Map<TypeVariable<?>, Type> typeArguments, final Type type) {
        if (typeArguments == null) {
            typeArguments = Collections.<TypeVariable<?>, Type> emptyMap();
        }
        if (containsTypeVariables(type)) {
            if (type instanceof TypeVariable<?>) {
                return unrollVariables(typeArguments, typeArguments.get(type));
            }
            if (type instanceof ParameterizedType) {
                final ParameterizedType p = (ParameterizedType) type;
                final Map<TypeVariable<?>, Type> parameterizedTypeArguments;
                if (p.getOwnerType() == null) {
                    parameterizedTypeArguments = typeArguments;
                } else {
                    parameterizedTypeArguments = new HashMap<>(typeArguments);
                    parameterizedTypeArguments.putAll(TypeUtils.getTypeArguments(p));
                }
                final Type[] args = p.getActualTypeArguments();
                for (int i = 0; i < args.length; i++) {
                    final Type unrolled = unrollVariables(parameterizedTypeArguments, args[i]);
                    if (unrolled != null) {
                        args[i] = unrolled;
                    }
                }
                return parameterizeWithOwner(p.getOwnerType(), (Class<?>) p.getRawType(), args);
            }
            if (type instanceof WildcardType) {
                final WildcardType wild = (WildcardType) type;
                return wildcardType().withUpperBounds(unrollBounds(typeArguments, wild.getUpperBounds()))
                    .withLowerBounds(unrollBounds(typeArguments, wild.getLowerBounds())).build();
            }
        }
        return type;
    }

    /**
     * Local helper method to unroll variables in a type bounds array.
     * 
     * @param typeArguments assignments {@link Map}
     * @param bounds in which to expand variables
     * @return {@code bounds} with any variables reassigned
     */
    private static Type[] unrollBounds(final Map<TypeVariable<?>, Type> typeArguments, final Type[] bounds) {
        Type[] result = bounds;
        int i = 0;
        for (; i < result.length; i++) {
            final Type unrolled = unrollVariables(typeArguments, result[i]);
            if (unrolled == null) {
                List<Type> types = Arrays.asList(result);
                types.remove(i--);
                result = types.toArray(new Type[types.size()]);
            } else {
                result[i] = unrolled;
            }
        }
        return result;
    }

    /**
     * Learn, recursively, whether any of the type parameters associated with {@code type} are bound to variables.
     *
     * @param type the type to check for type variables
     * @return boolean
     */
    public static boolean containsTypeVariables(final Type type) {
        if (type instanceof TypeVariable<?>) {
            return true;
        }
        if (type instanceof Class<?>) {
            return ((Class<?>) type).getTypeParameters().length > 0;
        }
        if (type instanceof ParameterizedType) {
            for (final Type arg : ((ParameterizedType) type).getActualTypeArguments()) {
                if (containsTypeVariables(arg)) {
                    return true;
                }
            }
            return false;
        }
        if (type instanceof WildcardType) {
            final WildcardType wild = (WildcardType) type;
            return containsTypeVariables(TypeUtils.getImplicitLowerBounds(wild)[0])
                || containsTypeVariables(TypeUtils.getImplicitUpperBounds(wild)[0]);
        }
        return false;
    }


    /**
     * Create a parameterized type instance.
     *
     * @param owner the owning type
     * @param raw the raw class to create a parameterized type instance for
     * @param typeArguments the types used for parameterization
     *
     * @return {@link ParameterizedType}
     */
    public static final ParameterizedType parameterizeWithOwner(final Type owner, final Class<?> raw,
        final Type... typeArguments) {
        Validate.notNull(raw, "raw class is null");
        final Type useOwner;
        if (raw.getEnclosingClass() == null) {
            Validate.isTrue(owner == null, "no owner allowed for top-level %s", raw);
            useOwner = null;
        } else if (owner == null) {
            useOwner = raw.getEnclosingClass();
        } else {
            Validate.isTrue(TypeUtils.isAssignable(owner, raw.getEnclosingClass()),
                "%s is invalid owner type for parameterized %s", owner, raw);
            useOwner = owner;
        }
        Validate.isTrue(raw.getTypeParameters().length == typeArguments.length,
            "invalid number of type parameters specified: expected %d, got %d", raw.getTypeParameters().length,
            typeArguments.length);

        return new ParameterizedTypeImpl(raw, useOwner, typeArguments);
    }

    /**
     * Get a {@link WildcardTypeBuilder}.
     * @return {@link WildcardTypeBuilder}
     */
    public static WildcardTypeBuilder wildcardType() {
        return new WildcardTypeBuilder();
    }

    /**
     * Check equality of types.
     *
     * @param t1 the first type
     * @param t2 the second type
     * @return boolean
     */
    public static boolean equals(final Type t1, final Type t2) {
        if (t1 == t2) {
            return true;
        }
        if (t1 == null || t2 == null) {
            return false;
        }

        if (t1.equals(t2)) {
            return true;
        }
        if (t1 instanceof ParameterizedType) {
            return equals((ParameterizedType) t1, t2);
        }
        if (t1 instanceof GenericArrayType) {
            return equals((GenericArrayType) t1, t2);
        }
        if (t1 instanceof WildcardType) {
            return equals((WildcardType) t1, t2);
        }
        return false;
    }

    /**
     * Learn whether {@code t} equals {@code p}.
     * @param p LHS
     * @param t RHS
     * @return boolean
     */
    private static boolean equals(final ParameterizedType p, final Type t) {
        if (t instanceof ParameterizedType) {
            final ParameterizedType other = (ParameterizedType) t;
            if (equals(p.getRawType(), other.getRawType()) && equals(p.getOwnerType(), other.getOwnerType())) {
                return equals(p.getActualTypeArguments(), other.getActualTypeArguments());
            }
        }
        return false;
    }

    /**
     * Learn whether {@code t} equals {@code a}.
     * @param a LHS
     * @param t RHS
     * @return boolean
     */
    private static boolean equals(final GenericArrayType a, final Type t) {
        return t instanceof GenericArrayType
            && equals(a.getGenericComponentType(), ((GenericArrayType) t).getGenericComponentType());
    }

    /**
     * Learn whether {@code t} equals {@code w}.
     * @param w LHS
     * @param t RHS
     * @return boolean
     */
    private static boolean equals(final WildcardType w, final Type t) {
        if (t instanceof WildcardType) {
            final WildcardType other = (WildcardType) t;
            return equals(getImplicitLowerBounds(w), getImplicitLowerBounds(other))
                && equals(getImplicitUpperBounds(w), getImplicitUpperBounds(other));
        }
        return false;
    }

    /**
     * Learn whether {@code t1} equals {@code t2}.
     * @param t1 LHS
     * @param t2 RHS
     * @return boolean
     */
    private static boolean equals(final Type[] t1, final Type[] t2) {
        if (t1.length == t2.length) {
            for (int i = 0; i < t1.length; i++) {
                if (!equals(t1[i], t2[i])) {
                    return false;
                }
            }
            return true;
        }
        return false;
    }

    /**
     * Present a given type as a Java-esque String.
     *
     * @param type the type to create a String representation for, not {@code null}
     * @return String
     */
    public static String toString(final Type type) {
        Validate.notNull(type);
        if (type instanceof Class<?>) {
            return classToString((Class<?>) type);
        }
        if (type instanceof ParameterizedType) {
            return parameterizedTypeToString((ParameterizedType) type);
        }
        if (type instanceof WildcardType) {
            return wildcardTypeToString((WildcardType) type);
        }
        if (type instanceof TypeVariable<?>) {
            return typeVariableToString((TypeVariable<?>) type);
        }
        if (type instanceof GenericArrayType) {
            return genericArrayTypeToString((GenericArrayType) type);
        }
        throw new IllegalArgumentException(type.toString());
    }


    /**
     * Format a {@link Class} as a {@link String}.
     * @param c {@code Class} to format
     * @return String
     */
    private static String classToString(final Class<?> c) {
        final StringBuilder buf = new StringBuilder();

        if (c.getEnclosingClass() != null) {
            buf.append(classToString(c.getEnclosingClass())).append('.').append(c.getSimpleName());
        } else {
            buf.append(c.getName());
        }
        if (c.getTypeParameters().length > 0) {
            buf.append('<');
            appendAllTo(buf, ", ", c.getTypeParameters());
            buf.append('>');
        }
        return buf.toString();
    }

    /**
     * Format a {@link TypeVariable} as a {@link String}.
     * @param v {@code TypeVariable} to format
     * @return String
     */
    private static String typeVariableToString(final TypeVariable<?> v) {
        final StringBuilder buf = new StringBuilder(v.getName());
        final Type[] bounds = v.getBounds();
        if (bounds.length > 0 && !(bounds.length == 1 && Object.class.equals(bounds[0]))) {
            buf.append(" extends ");
            appendAllTo(buf, " & ", v.getBounds());
        }
        return buf.toString();
    }

    /**
     * Format a {@link ParameterizedType} as a {@link String}.
     * @param p {@code ParameterizedType} to format
     * @return String
     */
    private static String parameterizedTypeToString(final ParameterizedType p) {
        final StringBuilder buf = new StringBuilder();

        final Type useOwner = p.getOwnerType();
        final Class<?> raw = (Class<?>) p.getRawType();
        final Type[] typeArguments = p.getActualTypeArguments();
        if (useOwner == null) {
            buf.append(raw.getName());
        } else {
            if (useOwner instanceof Class<?>) {
                buf.append(((Class<?>) useOwner).getName());
            } else {
                buf.append(useOwner.toString());
            }
            buf.append('.').append(raw.getSimpleName());
        }

        appendAllTo(buf.append('<'), ", ", typeArguments).append('>');
        return buf.toString();
    }

    /**
     * Format a {@link WildcardType} as a {@link String}.
     * @param w {@code WildcardType} to format
     * @return String
     */
    private static String wildcardTypeToString(final WildcardType w) {
        final StringBuilder buf = new StringBuilder().append('?');
        final Type[] lowerBounds = w.getLowerBounds();
        final Type[] upperBounds = w.getUpperBounds();
        if (lowerBounds.length > 1 || lowerBounds.length == 1 && lowerBounds[0] != null) {
            appendAllTo(buf.append(" super "), " & ", lowerBounds);
        } else if (upperBounds.length > 1 || upperBounds.length == 1 && !Object.class.equals(upperBounds[0])) {
            appendAllTo(buf.append(" extends "), " & ", upperBounds);
        }
        return buf.toString();
    }

    /**
     * Format a {@link GenericArrayType} as a {@link String}.
     * @param g {@code GenericArrayType} to format
     * @return String
     */
    private static String genericArrayTypeToString(final GenericArrayType g) {
        return String.format("%s[]", toString(g.getGenericComponentType()));
    }

    /**
     * Append {@code types} to @{code buf} with separator {@code sep}.
     * @param buf destination
     * @param sep separator
     * @param types to append
     * @return {@code buf}
     */
    private static StringBuilder appendAllTo(final StringBuilder buf, final String sep, final Type... types) {
        if (types.length > 0) {
            buf.append(toString(types[0]));
            for (int i = 1; i < types.length; i++) {
                buf.append(sep).append(toString(types[i]));
            }
        }
        return buf;
    }

}